GALLS 183 



species of Exoascus) which confine their attacks to the 

 epidermal cells and outer layers of ground-tissues. Some of 

 the latter fungi also cause the conspicuous " birds' nests " 

 referred to above. Cup-galls are formed, though of only 

 small size, by some inconspicuous species of Synchitrium (e.g. 

 on the Wood Anemone). The line of separation between 

 simple and compound galls is very ill-defined in the case of 

 the majority of Mycocecidia. 



NEMATO-CECIDIA, or, preferably, HELMINTHO-CECIDIA, 

 are the work of small nematoid worms. The great majority 

 of animals of this low group live in water or damp soil or in 

 decaying substances ; but a few, belonging to the genera 

 Tylcnclius and Heterodera, form more or less well-defined 

 galls on plants. The gall-making species H. SchacJitii is a 

 most formidable pest owing to the injury caused to many 

 cultivated plants by its tuberous galls on their roots. It has 

 been found over a great part of the earth's surface. The 

 females become mere swollen egg-bags. 



The species of Tylenchus are rather numerous, and their 

 galls are produced on almost all members of plants upwards 

 to the ovary. Their galls are swellings, often ill-defined, 

 but usually confined in each species to one special member, 

 e.g. on leaves of Agrostis and of Plantago, in ovary of wheat 

 (" corn-cockles "), and so on. They are mere enlargements 

 of the cellular tissues of the part, with no increased com- 

 plexity of structure. 



PHYTOPTO-CECIDIA are produced by a family of micro- 

 scopic mites characterised by their small cephalo-thorax and 

 long ringed abdomen, and by having only four small w r eak 

 legs. They have been in recent years studied by Dr. A. 

 Nalepa, who has published a scries of very valuable papers 

 on them, illustrated by most careful figures, and by Dr. 

 Canestrini in his " Prospetto del Acarofauna italiana." 



Dr. Nalepa has distinguished several genera and many 

 species of these mites, and Dr. Canestrini has also described 

 numerous species, while a few have been described by other 

 workers. Careful investigations of the mites themselves has 

 seldom revealed a species producing galls on plants of more 

 than one natural order ; but a reference is due to a paper by 

 Dr. J. Peyritsch (in " Sitzungsber. d. Kais. Akad. der 



